1. Field of the Invention
This invention relates to regenerable sorbents for use in desulfurization applications in fluidized beds, particularly in transport reactors. More particularly, this invention relates to sorbents that are resistant to attrition and that maintain chemical reactivity under the conditions of desulfurization applications in such reactors.
2. Description of Related Art
When coal is gasified, sulfur that is present in most coals is converted to sulfur bearing pollutant gases, in particular, hydrogen sulfide (H2S). When coal is burned, sulfur that is present in most coals is converted to sulfur dioxide (SO2) which, unless otherwise removed, is emitted with the coal combustion flue gases.
Development of a sorbent in a form suitable for removal of hydrogen sulfide from coal derived fuel gases, as well as for removal of sulfur dioxide from coal combustion flue gases, has been a challenging problem because of the requirements imposed for successful operation of sulfur removal systems. The sorbents should be operational at temperatures of 370xc2x0 to 750xc2x0 C. in the presence of reducing gases such as hydrogen and carbon monoxide. To obtain the necessary contact between the sulfur containing gas streams and the sorbents, the sorbents are typically incorporated into pellets or other particles suitable for use in fluid bed/transport reactors.
In use, sorbent particles undergo repeated cycles of sulfur absorption and regeneration which, in turn, results in either rapid and continuous decrease in reactivity or physical deterioration due to spalling. These deficiencies are thought to result from the decrease in surface area of the high surface area material in which the reactive oxide is disposed, which decrease occurs as a consequence of prolonged exposure to high temperatures. Internal porosity also decreases, thereby reducing surface area available for contact by the sulfur containing gas stream with the reactive oxide. Accordingly, it is apparent that there is a need for alternative sorbent preparations which overcome the disadvantages of conventional sorbent preparation techniques and provide sorbents with high attrition resistance and chemical reactivity.
Two important characteristics desirable in sorbents are resistance to attrition and maintaining of chemical reactivity. Numerous techniques have been developed over the years which are designed to improve the attrition resistance of sorbent particles. However, in many cases, these techniques are not suitable for the particular materials used to form particles for a given application. In addition, many of these techniques are limited to use with particular particles sizes. Conventional sorbent preparation techniques, such as coprecipitation and solid oxide mixing followed by granulation or spray drying are generally not suitable for preparing sorbents having attrition resistance and chemical reactivity that meet the stringent requirements of desulfurization applications in fluidized beds and transport reactors. These techniques require very high thermal treatment temperatures to impart physical strength and, as a result, they often produce sorbents with modest reactivity. This appears to be particularly true for sorbents based on zinc oxide (ZnO).
Research on the development of regenerable sorbents based upon ZnO has been extensive. Briefly, work prior to 1994 focused on extending the applicability of zinc-based sorbents to higher and higher temperatures. However, because of the tendency of ZnO to reduce and the higher vapor pressures of metallic zinc at these high temperatures, losses of zinc were reported. This problem was addressed through the compounding of ZnO with titanium oxide (TiO2), resulting in improvement in temperature operability up to 725xc2x0 C. With the shift in interest to desulfurization temperatures in the range of 343 to 538xc2x0 C. (650 to 1100xc2x0 F.), several approaches have been taken to enhance the reactivity of zinc-based sorbents. One researcher, for example, indicated that the compounding of ZnO with TiO2 is accompanied by lower reactivity and stressed the need to investigate alternative supports. Researchers proceeded by imparting improvements to earlier versions of zinc-based sorbents that were originally developed for the higher temperature range. One such sorbent was developed by a granulation technique for fluidized-bed applications in the higher temperature range and was tested at a pilot-scale level. However, when tested under transport reactor mode conditions at 538xc2x0 C., this sorbent was found to suffer attrition problems. A spray-dried version of this sorbent was also reported to exhibit an excessive attrition rate. These results underscore the difficulty of improving sorbent attrition to meet transport reactor requirements while maintaining practical chemical reactivity at moderate temperature.
U.S. Pat. No. 5,494,880 and related U.S. Pat. No. 5,866,503 teach durable regenerable sorbent pellets for removal of hydrogen sulfide coal gas containing titania as a diluent, high surface area silica gel, and a binder. These materials are mixed, moistened and formed into pellets which are then dried and calcined. U.S. Pat. No. 5,972,835 teaches a method for producing fluidizable, substantially spherical particulate material having improved attrition resistance and an average particle size from about 100 to about 400 microns by spray drying a slurry comprising inorganic starting materials and an organic binder.
Accordingly, it is one object of this invention to provide a method for producing regenerable sulfur sorbents for fluid bed/transport reactor applications having attrition resistance and chemical reactivity required by desulfurization applications therein.
This and other objects of this invention are addressed by a method for producing regenerable sulfur sorbents in which a support material precursor is mixed with isopropanol and a first portion of deionized water at an elevated temperature to form a sol mixture. A metal oxide precursor comprising a metal suitable for use as a sulfur sorbent is dissolved in a second portion of deionized water, forming a metal salt solution. The metal salt solution and the sol mixture are mixed with a sol peptizing agent while heating and stirring, resulting in formation of a peptized sol mixture. The metal oxide precursor is dispersed substantially throughout the peptized sol mixture, which is then dried, forming a dry peptized sol mixture. The dry peptized sol mixture is then calcined. The resulting calcined material is then converted to particles.